1. Technical Field
The present invention relates generally to communication devices for coupling to an external communication medium; and, more particularly, it relates to a telephone line data access arrangement providing improved surge protection.
2. Related Art
Regulatory agencies throughout the world have established standards and regulations for connecting subscriber equipment to telephone networks. These regulations are intended to prevent damage to the telephone network and mitigate interference with other equipment also connected to the network. The regulations, however, often present difficult design challenges.
According to the applicable regulatory standards of various countries, electrical and electronic equipment imported to or sold in these countries must neither cause excessive electromagnetic interference (EMI) nor be unduly susceptible to it (EMS). Compliance is often demonstrated by performing electromagnetic compliance (EMC) tests to recognized standards. FCC Part 15 (for unlicensed personal communication service devices) is exemplary of the regulatory requirements imposed on radiated or EMI emissions in the United States.
Products that must be tested generally include all electrical and electronic appliances, equipment and installations containing electrical and/or electronic components that are likely to cause or be susceptible to electromagnetic disturbances. A product achieves EMC when it is able to operate satisfactorily in its intended operating environment without causing EMI to other electronic equipment, and is itself able to operate satisfactorily without being adversely affected by EMI from other equipment operating in the same environment. Failure to pass an EMC compliance test may result in unacceptable delays in reaching the market place, expensive retrofitting of EMI control measures and/or other design modifications, removal of the equipment from the market and monetary penalties.
In addition to stringent EMI requirements, subscriber equipment or data communications equipment (DCE), such as a data modem, is generally required to provide for some form of electrical isolation to prevent voltage surges or transients originating from the subscriber equipment from having a deleterious effect on the telephone network and vice versa. Electrical isolation also addresses potential problems associated with differences in operating voltages between a telephone line and the subscriber equipment. More particularly, telephone line voltages may vary widely across a given network, and often exceed the operating voltage of subscriber equipment. In the United States, 1,500 volt isolation is currently required. In other countries, the prescribed isolation may reach 3,000-4,000 volts.
In the United States, certain of the voltage surge immunity requirements are set forth in the FCC Part 68. As stated therein, a main purpose of the FCC Part 68 Harmonized Requirements is to provide for uniform standards for the protection of a telephone network from harms caused by the connection of terminal equipment and associated wiring. The FCC Part 68 delineates two types of mandatory surge requirements—Type A and Type B surges on the telephone interface, both metallic (or horizontal) and longitudinal (or vertical). As defined in Part 68, Type B surges contain much lower energy than Type A surges. The two types of surges thus involve different fail-safe criteria. Part 68 compliance requires that both sets of fail-safe criteria be met.
Part 68 defines longitudinal voltage as one half of the vector sum of the potential difference between the TIP connection and earth ground, and the RING connection and earth ground for the TIP, RING pair of 2-wire and 4-wire connections; and, additionally for 4-wire telephone connections, one half of the vector sum of the potential difference between the TIP 1 connection and earth ground and the RING 1 connection and earth ground for the TIP 1, RING 1 pair (where TIP 1 and RING 1 are the receive pair). Part 68 further defines metallic or horizontal voltage as the potential difference between the TIP and RING connections for the TIP, RING pair of 2-wire and 4-wire connections and additionally for 4-wire telephone connections, between the TIP 1 and RING 1 connections for the TIP 1, RING 1 pair (where TIP 1 and RING 1 are the receive pair).
Some communications circuits, such as modems, employ a data access arrangement (DAA) having line side circuitry that includes a telephone (or other) network interface and system side circuitry including a host system interface. The line side circuitry and system side circuitry are separated by a high voltage isolation barrier (e.g., an isolation transformer or capacitive signal paths). Incoming and outgoing TIP and RING signals are generally communicated between line side circuitry and the TIP and RING connections via a diode bridge. Among other functions, the diode bridge prevents electrical damage to the DAA in the event of inverted wires in an RJ-11 jack.
The term DAA generally refers to circuitry, which provides an interface between a public telephone network originating in a central office (CO) and a digital data bus of a host system. The DAA electrically isolates a modem or similar device from a phone line to control emissions of electromagnetic interference/radio frequency interference (EMI/RFI). In addition to electrical isolation, the DAA often develops a number of signals (e.g., a ring signal) for provision to subscriber equipment. The DAA may receive signals from the phone line through a telephone jack, such as a RJ-11 connection as used for standard telephones.
Various circuits have been used to provide surge immunity in such communication circuitry. In a one typical arrangement, the telephone network interface includes a pair of “EMI” capacitors coupled between each of the TIP and RING signal lines and chassis ground or “earth ground”. The EMI capacitors function to shunt relatively high frequency (e.g., 30 MHz -1 GHz) emissions to chassis ground in order to reduce EMI radiated emissions from the external telephone cord connected to the RJ11 jack of the DAA. The EMI capacitors may also be used to control longitudinal conducted emissions between TIP/RING and earth ground.
In addition, a varistor, such as a Metal Oxide Varistor (MOV), is often provided between the TIP and RING signal lines to provide protection from metallic/horizontal surges. The MOV is a voltage clamping device generally capable of absorbing very large currents without damage. By itself, a MOV is capable of holding typical telephone line surge voltages down to a level of approximately 400V peak, or at any other rated voltage specification.